BR0308848B1 - low alloy steel and production method thereof. - Google Patents

Abstract

A low alloy steel, characterized by consisting of, by mass %, C:0.2-0.55%, Si:0.05-0.5%, Mn:0.1-1%, S:0.0005-0.01%, O(Oxygen):0.0010-0.01%, Al:0.005-0.05%, Ca:0.0003-0.007%, Ti:0.005-0.05%, Cr:0.1-1.5%, Mo:0.1-1% and Nb:0.005-0.1%, and the balance Fe and impurities; and also characterized by the impurities whose contents are restricted to P </= 0.03% and N </= 0.015%; and further characterized by containing composites of inclusions of not greater than 7 mu m in major axis with appearance frequency of not less than 10 pieces of composites per 0.1mm<2> of the steel cross section, wherein the composite comprises an outer shell of carbonitride of Ti and/or Nb surrounding a nucleus of oxysulfide of Al and Ca. <??>The low alloy steel suppresses pitting caused by inclusions and suppresses SSC induced by pitting.

Description

Report of the Invention Patent for "LOW ALLOY STEEL AND METHOD OF PRODUCTION".

TECHNICAL FIELD

The present invention relates to a low alloy steel and more particularly to a low alloy steel with a strong pitting corrosion resistance in an acidic environment which can suppress pitting induced stress cracking. . This low-alloy steel is suitable for use as oil well and gas well casing and pipe material, as well as drill pipes, drill loops, and well drilling pumping rods, and, in addition, pipelines or pipes for petrochemical plants, because it has a strong resistance to pitting corrosion and stress corrosion cracking in a severely acidic environment.

The present invention also relates to a method of producing low alloy steel.

BACKGROUND OF THE INVENTION

Today, the harsh conditions of energy sources have increased the demand for crude oil and natural gas, including a large amount of corrosive gas such as hydrogen sulfide and carbon dioxide, the use of which has so far been intentionally avoided. .

Thus, materials for this purpose are required to provide increased resistance to pitting and stress corrosion cracking in order to meet the requirement for drilling, transport and storage in this acidic environment.

In addition, materials are required to provide greater strength in order to satisfy the requirement for deeper drilling, more efficient transport and lower drilling costs, although a high strength steel is more susceptible to cracking under sulfide stress. Therefore, higher strength steel is required to provide greater resistance to cracking under sulphide stress.

Hereinafter, it refers to stress corrosion cracking as "SCC" and sulfide stress cracking as "SSC", respectively, in this descriptive report.

The following studies and proposals have been designed to suppress pitting, SCC, and SSC corrosion that may occur in a low alloy steel product such as pipe and pipe.

To suppress pitting corrosion and pitting corrosion induced SCC, an attempt was made to produce steel without impurities. However, techniques for minimizing the level of impurity elements and removing inclusions using equipment such as a tundish heater have their own limits from both a technical and cost aspects of steel production.

In order to suppress SSC, steel products have so far been perfected by the metallographic method, such as (1) producing them with less impurities, (2) producing their microstructure rich in the martensitic phase, (3) producing their microstructure. fine granulation and (4) heat-treated to temper at high temperatures. However, coarse non-metallic inclusions in steel products may cause pitting corrosion, which can often induce SSC. Thus, steel products containing coarse non-metallic inclusions cannot always be satisfied with improvement in the above metallographic method.

Unexamined Japanese Patent Publication No. 2001-131698 showed that Ti carbonitride caused pitting corrosion and thus induced SSC. Most low alloy steel products contain Ti because Ti is often added to make them fine grained and increase their strength. Ti carbonitride itself is insoluble in an acidic environment and has high corrosion resistance and high electrical conductivity. However, when immersed in an aqueous solution, it acts as a cathodic site to promote corrosion of the surrounding steel matrix. Japanese Unexamined Patent Publication showed that pitting corrosion susceptibility largely depended on the size of precipitated Ti carbonitride and proposed a method of pitting corrosion suppression by reducing nitrogen content and removing inclusions using a distributor. - tundish heater pain. However, this proposal is not satisfactory to suppress pitting corrosion despite the high cost during steel production.

It is an object of the present invention, which has been elaborated in view of the aforementioned prior art, to provide an excellent low alloy steel in pitting corrosion resistance that can prevent the occurrence of pitting corrosion caused by inclusions and pitting. also avoid SSC induction.

Another object of the present invention is to provide a method of producing low alloy steel.

DESCRIPTION OF THE INVENTION

The subject of the present invention is low alloy steel (1) and a following production method (2).

Low alloy steel (1)

A low alloy steel consisting of,% by mass, C: 0.2-0.55%, Si: 0.05-0.5%, Mn: 0.1-1%, S: 0.0005-0 0.01%, O (Oxygen): 0.0010-0.01%, Al: 0.005-0.05%, Ca: 0.0003-0.007%, Ti: 0.005-0.05%, Cr: 0.1 -1.5%, Mo: 0.1-1% and Nb: 0.005-0.1%, and optionally at least one alloying element selected from V: 0.03-0.5%, B: 0 , 0001-0.005% and Zr: 0.005-0.10%, and Fe and impurities as balance; wherein the impurity content of P and N is P <0.03% and N <0.015%; characterized in that it contains composites of inclusions no larger than 7 μm in the main axis with an appearance frequency of not less than 10 composite pieces per 0.1 mm2 of the steel cross section, where the composite comprises an outer shell of Ti, Nb carbonitride and / or Zr surrounding an oxysulfide core of Al and Ca.

It is preferable that the S content be 0.0010-0.01%.

Production Method (2)

A method of producing a low alloy steel as defined in (1) above, characterized by cooling the steel at a rate of no more than 500 ° C / min from 1,500 ° C to 1,000 ° C during casting of the steel.

In the present specification, the invention relating to low alloy steels (1) above is referred to as "invention (1)", and the invention referring to production method (2) above is referred to as "invention ( 2)". Sometimes inventions (1) and (2) are collectively referred to as "the present invention".

The composite of inclusions was evaluated as follows:

A plurality of fields of vision were arbitrarily selected in the cross section of each test specimen. In each field of view, the number and major axes of the composites observed per unit area were measured, and the composite whose major axis was greater in each field of view was specified, where the major axis of the composite was defined as the greater distance between two arbitrary points at a boundary of a composite relative to the matrix.

Then an average major axis value of the specified composite is calculated by dividing the sum of the major axis value by the number of fields of view. As a result, find the mean value of the major major axes of composites in the cross section of a test specimen, which is referred to as the value of "the major major axis" in summary hereinafter.

In an attempt to achieve the above objective, the inventor has made a number of investigations into thin-form inclusion dispersion technologies that may lead to precipitation of a thin-composite inclusion. The inventor came up with an idea of first forming an Al and Ca oxysulfide nucleus and then precipitating a Ti, Nb and / or Zr carbonitride around the nucleus. The inventor performed several experiments under this idea and obtained the following conclusions (a) to (c). (a) Al and Ca oxysulfide acts as a core to absorb Ti, Nb

and Zr. Therefore, when Al and Ca oxisulphide is previously formed, Ti, Nb and / or Zr carbonitride can precipitate around the core, resulting in precipitation of a large number of fine composite inclusions, each of which has a shell. of Ti, Nb and / or Zr carbonitride surrounding the nucleus of an Al and Ca oxysulphide.

Hereinafter such composite inclusion is referred to as "Al-Ca oxysulfide core carbonitride composite inclusion". Precipitation of the inclusion of Al-Ca oxysulfide core carbonitride composite may suppress precipitation of the coarse Ti, Nb and / or Zr carbonitride surrounding the Al oxide nucleus or the like, or may lead to precipitation of fine inclusions of carbon oxide. carbonitride not larger than 7 μm in size, even if the Ti, Nb and / or Zr carbonitride surrounding the Al-oxide core precipitates.

(b) Inclusion of precipitated fine carbonitride composite with Al-Ca oxysulfide core may not affect corrosion resistance.

(c) Inclusion of fine carbonitride composite with Al-Ca oxide sulphide core may be achieved by cooling at a rate of no more than

500 ° C / min from 1,500 ° C to 1,000 ° C during casting of the above low alloy steel (1). The inclusion of Al-Ca oxisulfide core carbonitride composite must have a major axis of at most 7 μm.

Based on the findings from (a) to (c) above, inventions (1) and (2) have been completed.

BRIEF DESCRIPTION OF THE INVENTION

Figure 1 is a typical example of the inclusion of Al-Ca oxysulfide core carbonitride composite having a major axis of no more than 7 μιτι.

Figure 2 is a schematic representation of EDX analysis sites of an Al-Ca oxysulfide core carbonitride composite inclusion having a major axis of no more than 7 μm.

Preferred Modes for Carrying Out the Invention

In the following, the present invention is described in detail. The expression "%" for the content of each element means "% by mass".

(A) Chemical composition of steel

C: 0.2-0.55%

C is an efficient element in increasing hardening capacity and improving strength, and no less than 0.2% is required. Above 0.55%, however, leads to a high susceptibility to quench crack formation and also a decrease in toughness. Therefore, the C content should be 0.2-0.55%.

Si: 0.05-0.5%

Si is an element necessary for deoxidation and its content less than 0.05% is necessary to produce a satisfactory deoxidation effect.

Above 0.5%, however, reduces hardness and resistance to SSC. Therefore, the Si content should be 0.05-0.5%. A preferred content range is 0.05-0.35%.

Mn: 0.1-1%

Mn is an element which has an effect of increasing the hardening capacity of steel and, in order to achieve this effect, a content of not less than 0,1% is required. Above 1%, however, increases Mn segregation at grain boundaries, which decreases hardness and resistance to SSC. Therefore, the Mn content should be 0.1-1%. A preferred content range is 0.1-0.5%.

S: 0.0005-0.01%

S, together with Ca, Al and O (oxygen), forms a thin core of Al and Ca oxysulfide that leads to precipitation of Ti, Nb and / or Zr carbonitride around the nucleus, which results in precipitation of an inclusive - are made of fine carbonitride composite with Al-Ca oxysulfide core. This inclusion of fine composite has the effect of suppressing the formation of a coarse carbonitride of Ti, Nb and / or Zr. In order to achieve this effect, an S content of not less than 0.0005% is required. Above 0.01% S, however, decreases pitting and SSC corrosion resistance. Therefore, the S content should be 0.0005-0.01%. A preferred content of S is 0.0010-0.01%.

O (oxygen): 0.0010-0.01%

O, together with Ca, Al and S, forms a thin core of Al and Ca oxisulphide that leads to Ti, Nb and / or Zr carbonitride precipitation around the nucleus, which results in precipitation of an inclusion of fine carbonitride composite with Al-Ca oxysulfide core. This fine composite has the effect of suppressing the formation of a coarse carbonitride of Ti, Nb and / or Zr. In order to obtain this effect, O content not less than 0.0010% is required. Above 0.01%, however, decreases pitting and SSC corrosion resistance; therefore, the O content should be 0.0010-0.01%. Al: 0.005-0.05%

Al is a necessary element for steel deoxidation and, when its content is below 0.005%, this effect can hardly be obtained. On the other hand, this effect saturates at a level exceeding 0.05%, and in addition, coarse Al-based oxides form abundantly, leading to a decrease in hardness. In addition, Al, together with Ca, S and O, forms a thin core of Al and Ca oxysulfide that leads to Ti, Nb and / or Zr carbonitride precipitation around the nucleus, which results in precipitation of an inclusion of Al-Ca oxysulfide thin carbonitride composite. This fine composite has the effect of suppressing the coarse carbonitride formation of Ti, Nb and / or Zr, so the Al content should be 0.005-0.05%. The term "Al" as used herein denotes "soluble Al" which means acid soluble Al.

Ca: 0.0003-0.007%

Ca is an important element in the practice of the present invention.

Ca, together with Al, S and O, forms a thin core of Al and Ca oxysulfide that leads to precipitation of Ti, Nb and / or Zr carbonitride around the core, which results in precipitation of a composite inclusion of fine carbonitride with Al-Ca oxysulfide core. And the fine composite has the effect of suppressing the coarse carbonitride formation of Ti, Nb and / or Zr. In addition, the thin composite improves pitting, SCC and SSC corrosion resistance. If the Ca level is below 0.0003%, however, the effect of the addition is poor. If the Ca level exceeds 0.007%, on the other hand, Al and Ca oxisulfide itself becomes coarse, which causes pitting corrosion. Therefore, the Ca content should be 0.0003-0.007%.

Ti: 0.005-0.05%

Ti absorbs carbon and nitrogen in the steel around an Al and Ca oxysulfide core, resulting in precipitation of a fine carbonitride composite inclusion with Al-Ca oxysulfide core. This is effective in making steel resistant by fine-graining crystals and conferring precipitation resistance. Additionally, in boron containing steel, Ti is effective in suppressing the formation of boron nitride, which results in enhancing the hardening ability due to B. To achieve these effects, Ti content of not less than 0.005% is required. On the other hand, over 0.05% Ti forms coarse Ti, Nb and / or Zr carbonitride, which may cause pitting corrosion, even though the Ca content is in the range mentioned above. Therefore, the Ti content should be 0.005-0.05%. A preferred content range is 0.005-0.03%. Cr: 0.1-1.5%

Cr improves hardenability and also increases hardening resistance by tempering, thus allowing for high temperature tempering and improved SSC resistance. These effects can be obtained if the Cr content is not less than 0.1%. However, if the Cr content level exceeds 1.5%, the above effects saturate and the cost increases. Therefore, the Cr content should be 0.1-1.5%. Mo: 0.1-1%

Mo improves hardenability and also increases hardening resistance by tempering, thus allowing high temperature tempering and improved SSC resistance. At levels below 0.1%, however, no satisfactory effect can be obtained. On the other hand, if the Mo content level exceeds 1%, Acicular Mo carbide precipitates during quenching, causing hardness decrease and SSC resistance. Therefore, the Mo content should be 0.1-1%.

Nb: 0.005-0.1%

Nb absorbs carbon and nitrogen in the steel around the Al and Ca oxysulfide core, resulting in precipitation of a fine carbonitride composite inclusion with Al-Ca oxysulfide core. This is effective in making steel resistant by fine-graining crystals and conferring precipitation resistance.

When its content is less than 0.005%, the addition effect is poor. On the other hand, under grade levels exceeding 0.1%, the above effect saturates and the cost increases. Therefore, the Nb content should be 0.05-0.1%.

The contents of the impurity elements PeN are restricted as mentioned below. Ρ: not more than 0,03%

P inevitably exists as impurity in steel. It is actively dissolved and thus reduces pitting corrosion resistance. It also segregates into grain boundaries, causing decreased hardness and resistance to SSC. In particular, when its content exceeds 0.03%, it decreases in hardness and resistance to pitting and SSC corrosion. Therefore, the P content should be no more than 0.03%. It is desirable that the P content be as low as possible.

N: Not more than 0.015%

N is an element that inevitably exists as impurity in steel. If N exceeds 0.015%, it will not lead to precipitation of an Al-Ca1 oxysulfide core fine carbonitride composite but will lead to precipitation of a coarse Ti, Nb and / or Zr carbonitride which may cause pitting corrosion. Therefore, the N content should be no more than 0.015%. It is desirable for the N content to be as low as possible.

A low alloy steel according to the invention (1) essentially satisfies the above mentioned chemical composition, and optionally further contains one or more elements selected from the elements V, B and Zr mentioned below in addition to the above mentioned chemical composition. . V, B or Zr contributes to the improvement of steel strength.

V: 0.03-0.5%

V could be added. If added, however, it would precipitate a fine carbide during quenching and thereby increase quenching resistance, whereby quenching at high temperatures is possible and SSC resistance is improved. To ensure this effect, the V content is desirably not less than 0.03%. On the other hand, if its content level exceeds 0.5%, the above effect saturates and the cost increases. Therefore, when added, the V content is preferably 0.03-0.5%.

B: 0.0001-0.005%

B could be added. When added, however, it is effective, even in trace amounts, in improving the hardening capacity of steel. To ensure this effect, the B content is preferably not less than 0.0001%. On the other hand, B above 0.005% leads to the precipitation of a coarse carbobide along grain boundaries, leading to decreased hardness and resistance to SSC. Therefore, when added, the B content is preferably 0.001-0.005%, more preferably 0.0001-0.003%. Zr: 0.005-0.10%

Zr could be added. When added, however, it absorbs carbon and nitrogen in the steel around the Al and Ca oxysulfide core, which leads to Ti, Nb and / or Zr carbonitride precipitation around the core, resulting in precipitation of a inclusion of fine carbonitride composite with Al-Ca oxysulfide core. Also, it is effective in increasing strength by thinning crystal grains and conferring precipitation resistance, and additionally in enhancing the hardening capacity due to B. To ensure these effects, the content of Zr is preferably not less than 0.005%. On the other hand, above 0.10% Zr forms a coarse carbonitride of Ti, Nb and / or Zr, which may cause pitting corrosion, even though the Ca content is in the range mentioned above. Therefore, when added, the Zr content is recommendably 0.005-0.10%. (B) Inclusion of Al-Ca oxysulfide core carbonitride composite in steel

The inclusion of Al-Ca oxide sulphide carbonitride composite in the low alloy steel according to the invention provides an outer shell of Ti, Nb and / or Zr carbonitride surrounding a core of an Al-oxisulphide. Ca. It is necessary that the carbonitride composite is not greater than 7 pm on the main axis with an appearance frequency of not less than 10 composite pieces per 0.1 mm2 of the steel cross section.

Al and Ca oxysulfide may contain oxysulfides of elements other than Al and Ca, equivalent to less than 50% of the total. Ti, Nb and / or Zr carbohydride may contain carbonitrides of elements other than Ti, Nb and / or Zr, equivalent to less than 50% of the total. Al oxide immediately aggregates and becomes coarse, so it is ineffective in producing fine dispersions. Therefore, it leads to a coarse carbonitride of Ti, Nb and / or Zr. In contrast, Al and Ca oxisulfides hardly aggregate, so they are effective in producing fine dispersions. Therefore, they may be a core to form a Ti, Nb and / or Zr1 carbonitride which leads to precipitation of a finely dispersed Ti, Nb and / or Zr carbonitride surrounding the core.

In addition, Ca is stronger in oxysulfide formation capacity than Al and therefore Al and Ca oxysulfide forms prior to Al oxide formation. Thus, an inclusion of Al oxysulfide core fine carbonitride composite -Ca having an outer shell of Ti, Nb and / or Zr1 carbonitride surrounding an Al and Ca1 oxysulfide core suppresses formation of a coarse Ti, Nb and / or Zr carbonitride surrounding an Al oxide core. pitting corrosion resistance is therefore improved.

However, if the inclusion of Al-Ca oxysulfide core carbonitride composite is itself gross, it causes pitting corrosion as well as coarse Ti, Nb and / or Zr carbonitride. In particular, when the mainshaft exceeds 7 pm, the decrease in pitting corrosion resistance is noticeable. Therefore, the maximum major axis in the inclusion of Al-Ca oxysulfide core carbonitride composite must be no more than 7 μm.

On the other hand, if the number of these composites is less than 10 by 0.1 mm2, the Al and Ca oxysulfide core cannot absorb Ti, Nb and / or Zr in the steel to a sufficient degree, even if the inclusion of composite Al-Ca oxysulfide core carbonitride is not greater than 7 μηι on the main axis. The unabsorbed portion of Ti, Nb and / or Zr forms a coarse carbon of Ti, Nb and / or Zr, which surrounds an Al oxide core, so that the resistance to small holes decreases. Therefore, the steel of the present invention should contain 10 or more pieces of the inclusion of Al-Ca oxysulfide core carbonitride composite by 0.1 mm 2. In assessing these inclusions, five fields of view are arbitrarily selected in the cross section of each test specimen. In each field of view, the number and major axes of the composites observed are measured by 0.1 mm2 and the composite whose major axis was greatest in each field of view is specified, where the main axis of the composite was defined. as the largest distance between two arbitrary points at a boundary of a composite relative to the matrix.

Then, the average major axis value of the specified composite is calculated by dividing the sum of the major axis value by 5, which is the number of fields of view. As a result, we find the value of the "major major axis", that is, the mean value of the major major axes of composites in the cross section of a test specimen.

The low alloy steel according to the invention (1) meets the above requirements for the inclusion of Al-Ca oxysulfide core carbonitride composite. It is also necessary to cool at a rate of no more than 500 ° C / min from 1,500 ° C to 1,000 ° C during casting to ensure sufficient time to allow Al and Ca oxysulfides to absorb Ti, Nb and Zr.

EXAMPLES

Fourteen species of low alloy steel, with their chemical compositions specified in Table 1, were melted.

Each steel species (150 tonnes) was continuously cast into round ingots having a diameter of 220 mm. At this time, the cooling rate, in the range of 1500-10000C, was varied, as shown in Table 2, by controlling the amount of mold cooling water and to cool ingots during 1500 ° casting. C at 1,000 ° C.

Thereafter, the round steel H and steel I ingots were each preheated to 1,250 ° C and then hot forged and hot rolled by conventional methods to produce 15 mm thick sheets.

The round steel A, steel C and JaM steel ingots were each reheated to 1,250 ° C and then hot rolled by the conventional method to produce round bars with a diameter of 40 mm.

The round steel ingots B, DaGe steel N steels were each reheated to 1,250 ° C and then hot rolled by the conventional method to produce seamless tubes with a wall thickness of 10 mm. Table 1

<table> table see original document page 15 </column> </row> <table> Table 2

<table> table see original document page 16 </column> </row> <table>

Note: * is outside the scope of the present invention.

Test specimens, 10 mm thick, 10 mm wide and 10 mm long, were cut from the plates, round bars and steel tubing thus obtained. They were embedded in a resin to reveal cross sections cut perpendicularly in the direction of hot rolling as test faces, and the test faces were mirror-polished and examined for inclusion by scanning electron microscopy under a Thus, each test face was observed in the five fields of view under a scanning electron microscope under an amplification of 200. Next, the number, observed by 0.1 mm2 in each field, of inclusions of Al-Ca oxysulfide core composite whose main axis was no larger than 7 μηη was counted and the mean calculated in five fields. In addition, the values of the "major major axis", that is, the average of the highest values in each major axis field of the inclusion of Al-Ca oxysulfide core composite and other carbonitrides were also measured. The inclusion of Al-Ca oxysulfide core composite was analyzed to determine its composition using an EDX (energy dispersion X-ray microanalyzer).

A typical example of the inclusion of Al-Ca oxisulfide core carbonitride composite, with a major axis of no more than 7 μm, is shown in Figure 1. The black core portion consists of Al and Ca oxysulfide and the white portion of the outer shell consists of Ti, Nb and / or Zr carbide.

Figure 2 is a schematic illustration of the EDX analysis sites of one of the Al-Ca oxysulfide core carbonitride composite inclusions. EDX analysis was performed at eight sites in total as shown in the figure.

Inclusion examination results are shown in Table 2, along with cooling rates between 1,500 ° C-1,000 ° C.

Then 3mm thick, 10mm wide and 40mm long corrosion test specimens were cut from the above steel plates, round bars and tubes, polished with ns 600 emery cloth and immersed in a solution. degassed aqueous solution containing 0.5% acetic acid and 5% sodium chloride at 25 ° C for 100 hours, and were then checked to determine whether or not pitting corrosion had occurred. The results of this investigation are also shown in Table 2.

Table 2 also shows that tests 1 to 7 and 14 satisfy the requirements prescribed in the present invention, and no pitting corrosion was also observed, so the corresponding steels also have good pitting corrosion resistance. Conversely, in tests number 8 to 13, pitting corrosion caused by coarse Ti, Nb and / or Zr carbide was observed. INDUSTRIAL APPLICABILITY

The low alloy steel of the invention suppresses pitting corrosion caused by inclusions and suppresses pitting corrosion induced SSC. Therefore, it can be used as casing and piping material for oil well and gas well, as well as drill pipes, drill collars and wellhead pumping rods, and additionally , pipes or tubes for petrochemical plants.

Claims (3)

1. Low alloy steel consisting of,% by mass, C: 0,2 to -0,55%, Si: 0,05 to 0,5%, Mn: 0,1 to 1%, S: 0,0005 0.01%, O (Oxygen): -0.0010 to 0.01%, Al: 0.005 to 0.05%, Ca: 0.0003 to 0.007%, Ti: 0.005 to 0.05%, Cr: 0.1 to 1.5%, Mo: 0.1 to 1% and Nb: 0.005 to 0.1%, and optionally at least one alloying element selected from V: 0.03 to 0.5% B: 0.0001 to 0.005% and Zr: 0.005 to 0.10%, and Fe and impurities as balance; where the impurity contents PeN are P ≤ 0,03% and N ≤ 0,015%; characterized in that it contains composites of inclusions of not more than 7 pm in the main axis with an appearance frequency of not less than 10 composite pieces per 0.1 mm2 of the steel cross section, the composite comprising an outer shell of carbonitride of Ti, Nb and / or Zr surrounding an oxysulfide core of Al and Ca. Low alloy steel according to claim 1, characterized by an S content of 0.0010 to 0.01%. Low alloy steel production method as defined in claim 1 or 2, characterized by cooling the steel at a rate of not more than 500 ° C / min from 1,500 ° C to 100,000 ° C during casting. steel.